In an optical communication system, coherent detection has various advantages in comparison with conventional intensity modulation direct detection or phase modulation delay interference detection. For example, using digital signal processing, coherent detection allows a low signal-to-noise ratio, compensates for a linear degradation, and achieves a high spectrum efficiency. Thus, coherent detection is expected to be widely spread in a next-generation optical communication system.
A coherent receiver mixes a received optical signal and local oscillator light before converting the received optical signal into an electrical signal. By so doing, a baseband signal indicating an electric field envelope of the optical signal is obtained. Then, transmission data is recovered from the baseband signal through digital signal processing.
However, it is difficult to make completely identical with each other the frequency of a transmitter laser light source to generate carrier light in an optical transmitter and the frequency of a local oscillator light source to generate local oscillator light in an optical receiver. Thus, the optical receiver compensates for a frequency difference (i.e., frequency offset) between the transmitter laser light source and the local oscillator light source so as to recover data. Currently, commercially available lasers have an oscillatory frequency error of ±2.5 GHz. Accordingly, in the aforementioned optical communication system, a frequency offset of ±5 GHz could be generated.
A method for superimposing a dithering signal on generated laser light by frequency modulation is known as one technology to stabilize the oscillation frequency of a laser light source. In this case, a frequency component of the dithering signal or a harmonic content thereof is detected from a generated optical signal, and the oscillation frequency is controlled according to the result of the detection. This allows the oscillation frequency of the laser light source to be locked at a desired value. The frequency of the dithering signal is sufficiently slow in comparison with the symbol rate of a transmission signal.
Methods for detecting frequency offset in a digital coherent receiver are described in, for example, Japanese Laid-open Patent Publication No. 2009-253971 and Japanese Laid-open Patent Publication No. 2011-228819.
An optical receiver that uses coherent detection and digital signal processing (hereinafter referred to as a “digital coherent receiver”) estimates a frequency offset and compensates for the frequency offset in accordance with the result of the estimation. Then, the digital coherent receiver obtains a modulated phase indicating code information from the received signal for which frequency offset has been compensated for and recovers transmission data from the modulated phase.
While a transmitter laser light source is being controlled using the aforementioned dithering signal in an optical transmitter, the frequency of a carrier signal varies in accordance with the dithering signal. In this case, frequency offset also varies in accordance with the dithering signal. Thus, when a dithering signal is used in an optical transmitter, a digital coherent receiver preferably performs frequency offset estimation (and frequency offset compensation) at a high speed so that the dithering signal can be followed. When a dithering signal is used in an optical transmitter and frequency offset estimation is performed at a low speed, frequency offset from is not properly compensated for, thereby preventing a phase error of a received optical signal from being properly corrected. In this case, data is recovered according to a signal having a phase error, causing a risk of degradation of a bit error rate.
The phase of an optical signal may also be changed by another factor. For example, factors of a phase error include ASE (Amplified Spontaneous Emission), noise, dispersion, and waveform degradation caused by nonlinear effect in an optical transmission line between an optical transmitter and an optical receiver. However, the phase error caused by these factors may be suppressed by averaging. Thus, performing frequency offset estimation (and frequency offset compensation) at a low speed suppresses a phase error caused by those factors and improves a bit error rate. In other words, performing frequency offset estimation at a high speed does not suppress a phase error caused by, for example, ASE, noise, dispersion, and a nonlinear effect and does not improve a bit error rate.
Note that “Estimating frequency offset at a high speed” is achieved by, for example, estimating frequency offset according to a small number of received symbols. “Estimating frequency offset at a low speed” is achieved by, for example, estimating frequency offset according to many received symbols.
In such a situation, a digital coherent receiver is preferably one able to receive an optical signal from an arbitrary optical transmitter. However, a characteristic improvement by averaging is not obtained in a configuration in which frequency offset estimation is performed at a high speed. Meanwhile, a communication with an optical transmitter that uses a dithering signal may be degraded in a configuration in which frequency offset estimation is performed at a low speed.